In recent years, attention has been paid, for example, to the optical modulation system such as the differential binary phase shift keying (DBPSK or DPSK) or the differential quadrature phase shift keying (DQPSK) as the technique for enabling high bit rate optical transmission of 40 Gb/s or more per wavelength.
As the requirement of the optical modulation system in the photonic network corresponding to high bit rate, it is expected to have the excellent characteristics for the technical items listed up, for example, in regard to optical noise tolerance, chromatic dispersion tolerance, polarization mode dispersion tolerance, optical non-linear tolerance, OADM filter passing tolerance and transceiver size/cost or the like. Particularly, the system ensuring optical noise tolerance and chromatic dispersion tolerance can be said to be more suitable for long-range optical communication. Moreover, the DQPSK system explained above has been verified, from the result of simulation or the like, to show more excellent characteristics in regard to many factors of the technical elements listed up in comparison with the well known ordinary non-return-to-zero (NRZ) modulation, carrier-suppressed return-to-zero (CS-RZ) modulation system, and DPSK modulation system.
As a practical example, FIG. 16 illustrates a comparison result in each modulation system of the optical noise tolerance, chromatic dispersion tolerance and PMD tolerance in regard to the optical modulation system of NRZ, Duo-binary, CS-RZ, RZ-DPSK, and RZ-DQPSK. Moreover, FIG. 17 illustrates simulation result in regard to the passing tolerance of wavelength combining/branching filters (for example, OADM).
The DQPSK modulation system in this specification is assumed to include the RZ-DQPSK system in which the DQPSK signal is converted to the return-to-zero (RZ) pulse in its intensity waveform and the carrier-suppressed (CS) RZ-DQPSK system. Moreover, the technique disclosed in this specification can be adapted to the differential M-phase shift keying system including the DMPSK (M=2n) such as D8PSK.
Here, an optical transmitter and an optical receiver introducing the DQPSK system will be explained briefly.
As the optical transmitter introducing the DQPSK system, the optical transmitter having the basic structure illustrated, for example, in FIG. 18 is known (for example, patent document 1). In this optical transmitter, a continuous light emitted from a light source 500 is branched into two light beams and one branched light is applied to a phase modulator (PM) 510, while the other branched light to a phase modulator (PM) 513 via a delay unit 512. Each phase modulator 510, 513 is independently driven in accordance with the modulation signal generated by processing different data signals D1, D2 with the process in a pre-coder (integrator) 531 in order to selectively change the phase of each input light by 0 or π [radian]. Details of pre-coder and modulation are explained in the patent document 1. Since the input light to the phase modulator 513 is given a phase difference as much as π/2 by the delay unit 512 for the input light to the phase modulator 510, an output light from the phase modulator 510 becomes the light signal obtained by modulating the light from the light source 500 through phase shift of 0 or π, while an output light from the phase modulator 513 becomes the light signal obtained by modulating the light from the light source 500 through phase shift of π/2 or
3π/2. Since the output lights of the phase modulators 510, 513 are combined, the DQPSK signal light which change in the phase in the four values of π/4, 3π/4, 5π/4, and 7π/4 can be generated. Since a bit rate of this DQPSK signal light becomes two times the bit rate of each data signal D1 or D2, it is enough, for transmission of the DQPSK signal light, for example, of 40 Gb/s, to process the data signals D1, D2 of 20 Gb/s with the pre-coder and drive each phase modulator 510, 513.
Moreover, as illustrated in an example of structure, for example, of FIG. 18, the RZ-DQPSK signal light is generated by giving the DQPSK signal light to an intensity modulator 540 which is driven with the clock signal CLK synchronized with the data signals D1, D2 in order to obtain the RZ pulse, while the CSRZ-DQPSK signal light in the duty ratio of about 67% can be generated by setting the frequency of the clock signal CLK to ½ of the data signals D1, D2 and the amplitude thereof to a half-wavelength voltage (Vπ) of the intensity modulator 540. Light intensity and phase of the RZ-DQPSK signal have the relationship illustrated, for example, in FIG. 19. In the figure, light intensity changes periodically as a result of the RZ modulation with CLK, whereas light intensity becomes constant when only the DQPSK modulation is carried out.
As an optical receiver of the background art for demodulating the DQPSK signal light, a receiver in the structure, for example, as illustrated in FIG. 20 is known (for example, refer to Japanese Published Application No. 2004-516743). In this optical receiver, the input DQPSK signal light is branched into two signal lights. Each branched light is respectively given to delay interferometers 501, 502. The delay interferometers 501, 502 are structured to generate a relative delay time difference corresponding to approximately one symbol of the DQPSK-modulated code between the signal lights propagated in each arm by giving difference in the lengths of optical paths of two arms of a Mach-Zehnder type optical waveguide formed, for example, on a silica substrate or indium phosphate substrate. Moreover, an interference operating point of the delay interferometer 501 is set to π/4 with the delay unit 503 formed on one arm, while the interference operating point of the delay interferometer 502 is set to −π/4 with the delay unit 503 formed on the other arm. Complementary two outputs outputted from an output coupler of the delay interferometer 501 are received with a differential receiving circuit 505 formed of a pair of optical detectors and an electric amplifier and thereby an electrical signal A corresponding to the signal D1 inputted to the transmitter is demodulated. Moreover, in the similar manner, complementary two outputs outputted from an output coupler of the delay interferometer 502 are also received with a differential receiving circuit 506 formed of a pair of optical detectors and an electric amplifier and thereby, an electrical signal B corresponding to the signal D2 inputted to the transmitter is generated through demodulation. The electric signals A/B are regenerated as stable electric signals by CDR (clock and data recovery) circuits and are thereafter subjected to the frame synchronization processes such as SDH/SONET/OTN or the like, regeneration of frame by a framer circuit and error correction by an FEC decoder circuit.
Moreover, the delay interferometers used for the optical receiver of the background art is also known in the structure combining, for example, fused optical fiber couplers in addition to the structure of optical waveguide type. See also the non-patent document by A. H. Gnauck et al., “Spectrally Efficient (0.8 b/s/Hz) 1−Tb/s (25×42.7 Gb/s) RZ-DQPSK Transmission Over 28 100-km SSMF Spans With 7 Optical Add/Drops”, ECOC2004, PD.4.4.1
However, the optical receiver of the background art as illustrated in FIG. 20 has a problem that two delay interferometers including rather long optical paths for delaying the time of one symbol must be provided and thereby the optical receiver tend to be large in size. More specifically, for demodulation of the DQPSK signal light, for example, of 40 Gb/s, a delay time difference of about 50 ps corresponding to one symbol of the data signal of 20 Gb/s must be generated within each delay interferometer and it is therefore required to provide difference in the light path of about 15 mm between the arms. In the case where such delay interferometer is realized with an optical waveguide formed on the silica substrate or the like, a pair of optical waveguide circuits of large area must be allocated and therefore it is impossible to avoid enlargement of the optical receiver. Moreover, in the optical receiver of the background art, since the interference operating point (i.e. optical phase) of one delay interferometer must be set accurately to π/4, while the operating point of the other delay interferometer must be set, also accurately, to −π/4, here rises also a problem that a technology is required to control optical phase with high accuracy between the delay interferometers.